1. Field of the Invention
The present invention relates to a surface acoustic wave filter having resonators with transverse coupling utilizing the coupling of three acoustic pathways. Surface acoustic wave filters are used in numerous applications, in particular in portable telephones.
2. Discussion of the Background
Such filters utilize the propagation of acoustic waves over the surface of a piezoelectric substrate, and the filtering characteristics stem from the modifications made to the propagation of these waves as a function of their frequency by electrodes suitably arranged on the surface of the substrate. There are several structures for these electrodes and a choice is made between them depending on the results to be obtained. Among these structures, those which utilize resonators transversely coupled by proximity are more particularly known.
In FIG. 1, the structure of the electrodes of a two-pole filter utilizing transverse-coupling resonators is represented. For the sake of clarity, the structure described here is very simple and in particular has a small number of fingers for the electrodes, which would give rise to relatively mediocre filtering performance of a filter made identically to this structure. However, this structure is very characteristic of filters of this type and practical implementations differ essentially through an increase in the number of fingers.
This two-pole filter consists of two resonators each formed of a transducer situated between two reflector arrays and coupled by proximity. The transducers and the reflector arrays have a metallization period of around half a wavelength. The first resonator is linked to the input of the filter and the second resonator to the output.
The input resonator comprises an electrode 1 in the shape of a comb whose fingers are interdigitated with the fingers of an earth electrode 2. The acoustic waves thus generated by this transducer are transmitted towards the right and towards the left towards reflectors 3 and 4 formed by fingers parallel to the fingers of the transducers and obtained by slitting the earth electrode 2. The resonator linked to the output is symmetric with this resonator linked to the input with respect to an axis parallel to the direction of propagation of the acoustic waves generated by the input resonator. It comprises a transducer 5 and two reflectors 6 and 7. With the earth being common, here the corresponding earth electrode 2 is in one piece and is linked here to two earth outputs 8 and 9 and solely for bonding purposes to the circuit outside the filter. It would be possible either to use just a single earth output or to separate the earths.
The two resonators are coupled by bringing them close together according to the layout of FIG. 1. Under these conditions, in first order of approximation the structure may be regarded as being equivalent to that obtained by bringing two waveguides close together. Each of the propagation modes of the guides considered on its own is transformed into two modes, a symmetric mode and an antisymmetric mode.
When the distance between the two resonators is large, the velocities of the acoustic waves in these two modes are very similar and substantially equal to the velocity of the mode of an isolated guide. The coupling is then very weak, this being all the more evident the further apart the resonators. As the distance between the two resonators decreases, the velocities of the waves in the two modes diverge and the coupling increases.
It can be shown that the equivalent diagram of such a structure corresponds to that obtained for two resonators coupled with a coupling equal to a normalized discrepancy between the frequencies f.sub.s and f.sub.a, of the two respectively symmetric and antisymmetric modes given by the following formula: ##EQU1##
This coupling thus defines the relative passband which can be obtained with this kind of filter.
In general, with a two-pole filter, too little rejection is obtained and the sides of the passband are too shallow. It is then expedient to produce filters with four or six poles, or even more quite simply by cascading several two-pole structures.
Represented in FIG. 2 is the structure of the filter previously described and illustrated by FIG. 1. The drawing is simplified by representing the elements by rectangles in which the fingers of the electrodes have not been depicted so as to make the representation more readable. The electrodes inside these rectangles would be drawn in accordance with the rules well known to a person skilled in the art.
The elements which are the counterpart of those of FIG. 1 are denoted by the same labels. This presentation will be retained for the other figures appended to the present description.
From this basic structure, illustrated in FIGS. 1 and 2, other known filter structures are produced. Among them, that illustrated in FIG. 3. This structure relates to a two-pole filter with differential inputs IN+, IN- and non-differential output OUT employing four acoustic pathways 10, 11, 12 and 13.
The symmetry of such a filter is obtained by paralleling two filters 14 and 15 of identical intrinsic structure but whose differential inputs have been inverted. In the structure of each of the two filters 14 and 15, the central bus 2 corresponding to the earth buses in the filter of FIGS. 1 and 2 is no longer common to the two poles but separated into two distinct buses: an earth bus 16, 17 and a bus 18, 19 linked to one of the differential inputs. To obtain low input impedance, the transducers are connected directly between the differential inputs. The inverting of the inputs is offset by inverting the polarities of the transducers. The drawback of such a structure resides in the width of the substrate required to produce the filter.